1. Field of Invention
The present invention relates to methods for crosslinking asphalt compositions and the improved crosslinked products resulting therefrom.
2. Discussion of the Background
The use of sulfur as an agent for crosslinking of SBR (Styrene Butadiene Rubber) and SBS (Styrene Butadiene Styrene block copolymers) in asphalt blends and the technical advantages derived from this process are well known in the industry. The main advantages are:
a) Polymer Stability
Both families of polymers provide an improvement in polymer stability, thereby preventing asphalt-polymer phase separation very efficiently.
b) Improvement in Blend Physical Properties
The weak physical properties obtained with SBR are highly improved by sulfur crosslinking. The use of non-crosslinked SBS in asphalt production immediately provides medium to strong physical properties. However, even in this particular case, the use of sulfur crosslinking produces a small to medium improvement in physical properties (particularly in softening point and penetration).
c) Low Temperature Performance
The use of crosslinking agents modifies the low temperature performance of the asphalt, making the asphalt more elastic at this condition. The resulting asphalt is softer, easier to deform and recovers more readily from applied stress at low temperatures. At the same time, at high temperature, the asphalt is not softer, provides better resistance to deformation and recovers more readily.
Sulfur is widely used as a crosslinking agent in SBR and low molecular weight, linear SBS, but is restricted with high molecular weight, radial SBS. This restriction on the use of sulfur as a crosslinking agent with radial, high molecular weight SBS is mainly due to the following:
High levels of sulfur addition are not allowed. The process is highly sensitive to sulfur concentration. Additions above 0.025% per each polymer percent produces gels with disastrous consequences, making the process too vulnerable in the event of a sulfur addition mistake.
Medium addition of sulfur in the order of 0.014 to 0.024% produces unpredictable results. Due to this unpredictability in this concentration range, the use of the same concentration of sulfur sometimes permits the process to run well, while at other times the polymer undergoes overcrosslinking and produces gels, thus increasing the risk when the concentration is higher but not eliminating the risk when the concentration is lower, in this medium range.
Low concentrations of sulfur in the order of 0.013% or lower per each percent of polymer is likewise unpredictable, with a given percentage sometimes producing good results, while at other times there is no reaction at all.
All these variations can occur unpredictably, even though the quality of the raw material and the process conditions remain constant. The variation is believed to be primarily related to a lack of repeatability inherent to the sulfur crosslinking process when it is used in radial, high molecular weight SBS.
To avoid these drawbacks, some producers use very low additions of 0.009% of sulfur or lower and set the polymer modified asphalt in storage at high temperature for more than 3 days. While this procedure avoids overcrosslinking, the most disastrous outcome, it still provides a high degree of variability in the resulting product. Low additions of sulfur, as low as 0.009%/% of polymer and a further storage up to three days at high temperature might reduce the overcrosslink risk but still delivers variable results.
Accordingly, one object of the present invention is to provide a process for crosslinking polymer modified asphalt compositions that gives improved reliability for the process and improved reproducibility.
A further object of the present invention is to provide a process for crosslinking polymer modified asphalt compositions that is easier to control and that provides a product that has a combination of properties that is similar to or better than conventional sulfur crosslinked asphalt compositions.
A further object of the present invention is to provide a crosslinked polymer modified asphalt composition that can be readily produced and has properties that are comparable to or even better than conventional sulfur crosslinked asphalt compositions.
These and other objects of the present invention have been satisfied by the discovery of a process for preparation of a crosslinked asphalt composition, comprising:
heating an asphalt composition at a first temperature of from 250xc2x0 F. (121xc2x0 C.) to 430xc2x0 F. (221xc2x0 C.), in the presence of a) a crosslinkable polymer and b) a crosslink co-agent;
adding a crosslinking initiator at a temperature of from 360xc2x0 F. (182xc2x0 C.) to 430xc2x0 F. (221xc2x0 C.); and
agitating the resulting mixture at a temperature at from 360xc2x0 F. (182xc2x0 C.) to 410xc2x0 F. (210xc2x0 C.) for a period of time sufficient to complete crosslinking,
and the crosslinked asphalt compositions resulting therefrom.